1. Field of the Invention
The present invention generally relates to light fixtures for illuminating architectural spaces. The invention has particular application in light fixtures using fluorescent lamps, such as the T5 linear fluorescent lamp, as the light source.
2. Background Art
Numerous light fixtures for architectural lighting applications are known. In the case of fixtures that provide direct lighting, the source of illumination may be visible in its entirety through an output aperture of the light fixture or shielded by elements such as parabolic baffles or lenses. A light fixture presently used in a typical office environment comprises a troffer with at least one fluorescent lamp and a lens having prismatic elements for distributing the light. Also known are light fixtures that use parabolic reflectors to provide a desired light distribution. The choice of light fixture will depend on the objectives of the lighting designer for a particular application and the economic resources available. To meet his or her design objectives, the lighting designer, when choosing a light fixture, will normally consider a variety of factors including aesthetic appearance, desired light distribution characteristics, efficiency, lumen package, maintenance and sources of brightness that can detract from visual comfort and productivity.
An important factor in the design of light fixtures for a particular application is the light source. The fluorescent lamp has long been the light source of choice among lighting designers in many commercial applications, particularly for indoor office lighting. For many years the most common fluorescent lamps for use in indoor lighting have been the linear T8 (1 inch diameter) and the T12 (1½ inch diameter). More recently, however, smaller diameter fluorescent lamps have become available, which provide a high lumen output from a comparatively small lamp envelope. An example is the linear T5 (⅝ inch diameter) lamp manufactured by Osram/Sylvania and others. The T5 has a number of advantages over the T8 and T12, including the design of light fixtures that provide a high lumen output with fewer lamps, which reduces lamp disposal requirements and has the potential for reducing overall costs. The smaller-diameter T5 lamps also permit the design of smaller light fixtures.
Some conventional fluorescent lamps, however, have the significant drawback in that the lamp surface is bright when compared to a lamp of larger diameter. For example, a conventional T5 lamp can have a surface brightness in the range of 5,000 to 8,000 footlamberts (FL), whereas the surface brightness of the larger T8 and T12 lamps generally is about 3,000 FL and 2,000 FL, respectively (although there are some versions of linear T8 and T12 lamps with higher brightness). The consequence of such bright surfaces is quite severe in applications where the lamps may be viewed directly. Without adequate shielding, fixtures employing such lamps are very uncomfortable and produce direct and reflected glare that impairs the comfort of the lighting environment. Heretofore, opaque shielding has been devised to cover or substantially surround a fluorescent lamp to mitigate problems associated with light sources of high surface brightness; however, such shielding defeats the advantages of a fluorescent lamp in regions of distribution where the lamp's surfaces are not directly viewed or do not set up reflected glare patterns. Thus, with conventional shielding designs, the distribution efficiencies and high lumen output advantages of the fluorescent lamp can be substantially lost.
A further disadvantage to traditional parabolic and prismatic troffers is the presence of distracting dynamic changes in brightness level and pattern as seen by a moving observer in the architectural space. Additionally, traditional parabolic and prismatic troffers allow direct or only slightly obscured views of the lamp source(s)) at certain viewing angles (low angles for both the parabolic and prismatic and most transverse angle for prismatic). This unaesthetic condition is remedied by indirect and direct-indirect fixture designs, but typically with a significant loss of efficiency.
Another known solution to the problem of direct glare associated with the use of high brightness fluorescent lamps is the use of biax lamps in direct-indirect light fixtures. This approach uses high brightness lamps only for the uplight component of the light fixture while using T-8 lamps with less bright surfaces for the light fixture's down-light component. However, such design approaches have the drawback that the extra lamps impair the designer's ability to achieve a desired light distribution from a given physical envelope and impose added burdens on lamp maintenance providers who must stock and handle two different types of lamps.
Conventional parabolic light fixture designs have several negative features. One of these is reduced lighting efficiency. Another is the so-called “cave effect,” where the upper portions of walls in the illuminated area are dark. In addition, the light distribution of these fixtures often creates a defined line on the walls between the higher lit and less lit areas. This creates the perception of a ceiling that is lower than it actually is. Further, when viewed directly at high viewing angles, a conventional parabolic fixture can appear very dim or, even, off.
The present invention overcomes the above-described disadvantages of light fixtures using brighter light sources by providing a configuration that appears to a viewer as though it has a source of lower brightness, but which otherwise permits the light fixture to advantageously and efficiently distribute light generated by the selected lamp, such as the exemplified T5 lamp. The light fixture of the present invention reduces distracting direct glare associated with high brightness light sources used in direct or direct-indirect light fixtures. This reduction in glare is accomplished without the addition of lamps and the added costs associated therewith.